I've read several posts here where people talk about matching their panel strings. Just how important is it and how do you go about doing it? I ask because out of curiosity, I decided to run a test by connecting just one string at a time to my upgraded STXR2500. I am running 8 Sharp 165w panels paired for 4 strings. I tried this test sort of late in the day, so I am not too sure how accurate the results are. I ran each string for several minutes to be sure the inverter reached the proper MPPT. These are the outputs I observed.

String #1 120w
String #2 116w
String #3 120w
String #4 146w

Now assuming these readings are accurate, string #4 has a higher output. If matching is important, it would seem that finding the right combination between string #2 and #4 would be called for. Is that correct or do you try to match panels to get the highest ouput from a single string?

The key to module matching is matching within each string. PV cells are essentially current sources with current proportional to insolation. As an example of matching, if four modules are connected in series and three of the modules produce 6 amperes while the fourth module produces 5.5 amperes, the four module string will only produce 5.5 amperes -- dominated by the weakest current. The key is to wire strings of four each of which produce about the same amount of current. (If one is using a SunnyBoy with one 24 modules string, the current is set by the weakest module and you can't do a thing about it.)

As for how to match, I did the following:
I took one module, exposed it to full sunlight (no clouds) and measured short-circuit current with a DMM. I then took and second module and placed it is the same postion exactly as the first and measured its short-circuit current, recording the result. I repeated the test on all 48 of my modules. After every six modules were tested, I returned the first module to the testing positon to correct for any changes in insolation. By the way, one must be very careful to avoid things like reflections off your shirt which will mess up your measurements. Given this data, I matched up the modules in groups of four where each module in a group is matched as closely as possible. Then I installed the PV modules wiring each circuit from matched sets.

Gordon

So, just how important is the matching? Are we talking about only milking a few extra watts out of the system or is there a greater gain? Since I only have 8 panels and they are already mounted, it would be easy to make individual measurements. However, I would not be thrilled with having to shuffle them around unless the gains were measurable.

Hi Mark,

It depends on how much the mismatch is and the luck of the draw in how they were randomly paired on you present installation.

Statistically, you might see 1/2 of the percentage differance between the worst and best panel.

One thing I would like to see would be what would happen in a long single string like on a Sunny Boy where one panel is 10% lower than the others. Would the current drop to the 90% level or would the current stay the same and the higher current panels reverse bias the weak panel and the voltage (not the current) drop by 17 volts for the string?

Ron

Ron

Mark,

My experience with 48 SP75s was a bit greater than a 3% difference in current from strongest to weakest.

If you can get to the boxes on the back of your modules. You could test (on a clear day) by removing your branch circuit fuses and short-circuit current test your modules one at a time.

Gordon